A rolling mill known from German patent specification No. 635,015 has a driven back up ring enclosing rolls which driven by the friction force against the back up ring and a tube run round it with satellite motion. The tube placed on a cylindrical mandrel is pushed or pulled through the rolling mill. Rolls have such a shape and are disposed in relation to the axis of the tube so that while rolling it they reduce at the same time the outer diameter of the tube.
A disadvantage of the above described rolling mill is the loading of the tube while subjecting the same to a full rolling torque, which results from the lack of a spinning and separately driven head wherein bearings of rolls would be mounted. This causes a torsional deformation of the tube. Besides, rolls are loaded with the axial pressure force of metal.
A method of increasing the outer diameter of a tube, known from Polish patent specification No. 83,997, consists in that increasing the diameter of a tube is carried out in two stages. In the first stage through the tube an immovable extension plug is pulled or pushed. In the second stage walls of the tube are transversely rolled on the extention plug by a set of three or more rolls running round the tube with satellite motion. The relation of the angular velocity of the planetary cage and the back up ring is selected so that the torque moment acting on the tube is close to zero. A device known from Polish patent specification No. 83,997 has a mandrel fixed with one end to the body, whereas on the other end thereof a conical extension plug is situated. Around the plug three working rolls are disposed, which have the principal part of their working surface in a frusto-conical shape. The rolls are mounted on bearings immovably or in a self-aligning manner in the planetary cage and supported by the back up ring connected with a power transmission system. The back up ring drives working rolls and--by supporting them--reduces their deflection. The planetary cage is connected with a separate power transmission system, whereby both power transmission systems allow such an adjustment of their angular velocities so that the torsional moment acting on the tube oscillates near zero. Axes of working rolls lie on the surface of the cone having the top situated in line with the axis of the tube and directed opposite to the direction of its movement. The tube is surrounded by a set of push-in rolls having axes perpendicular to the axis of the tube.
A disadvantage of the above described method and device is a considerable limitation of possibilities of producing tubes with a small diameter and a thin wall, resulting from the limitation of the value of elongation and the association of the reduction of the thickness of the wall with the enlargement of the diameter. The thickness of the rolled tube depends on the position of the extension plug in relation to the rolls. As a result of insufficient rigidity of the rolling mill in the direction of its axis there are difficulties in obtaining high accuracy of the thickness of the wall of the tube. Besides, axial bearings of rolls of the said rolling mill are loaded with the pressure of metal having a considerable value, which has a disadvantageous effect on their life.